Marine antifreeze: winterizing boats

Raw-water vs closed cooling

General winterizing steps (check your manual)

1. Run the engine to temperature, then change the oil if your manual says to.
2. Flush the raw-water side with fresh water.
3. Draw non-toxic marine antifreeze through the raw-water intake until it exits the exhaust.
4. Drain or treat the fresh water plumbing.
5. Check the closed-system coolant strength with a refractometer.

Common mistakes

• Using toxic EG coolant in raw-water circuits.
• Skipping the closed-loop coolant check.

Why boats need winterizing

When water freezes it expands, and trapped water in an engine block, raw-water pump, heat exchanger or plumbing line can crack metal and split hoses. A boat that sits in freezing temperatures without protection can suffer expensive damage that may not show until spring. Winterizing replaces or protects that water with antifreeze, or drains it completely.

Closed cooling: checking engine coolant

1. With the engine cold, check the coolant level in the expansion tank.
2. Test the freeze protection with a refractometer or test strips.
3. If it's weak or due for replacement, drain and refill with the engine maker's specified coolant.
4. Inspect the heat exchanger hoses and clamps.
5. Record what was used and when.

Choosing a marine antifreeze

• Check the burst protection rating against your coldest expected temperature.
• Use a PG-based non-toxic product for raw-water and potable systems.
• Check the label for compatibility with your engine and plumbing materials.
• Avoid diluting it unless the label allows.
• Buy enough for every circuit: engine, generator, plumbing, heads.

Spring recommissioning

Before launch, flush the potable water system until the antifreeze taste and color are gone, check the raw-water impeller, and run the engine on the hose or in the water while watching for flow from the exhaust and a normal temperature. Collect antifreeze that comes out of the system where practical, and dispose of it according to local rules.

Common winterizing mistakes

• Forgetting the generator or air conditioning raw-water circuit.
• Diluting antifreeze with water left in the system. Drain as much as possible first.
• Using automotive EG coolant in raw-water circuits.

Raw-water inboards: the two methods

BoatUS guidance describes two approaches for raw-water-cooled engines. Either drain every cooling passage, including seacock bodies, sea strainers and water-lift mufflers, or circulate full-strength, non-toxic propylene glycol antifreeze with rust inhibitors. In the circulation method, you close the seacock, put the raw-water intake hose in a bucket of antifreeze, and run the engine at idle until antifreeze comes out of the exhaust. BoatUS notes the engine should reach operating temperature so the thermostat opens. Follow your engine manufacturer's procedure where it differs.

Outboards and sterndrives

Outboards usually drain themselves when tilted down, but trapped water can remain in low spots. Many owners flush with fresh water and follow the manufacturer's storage procedure, which may include fogging oil and fuel stabilizer. Sterndrives have drain points in the block, manifolds and drive. Your owner's manual lists them. Any water left in a cast-iron block can freeze and crack it.

Winterizing checklist (whole boat)

• Engine raw-water circuit (drained or antifreeze-filled)
• Closed-cooling coolant tested to the correct strength
• Generator raw-water circuit
• Fresh-water system, water heater (bypassed), and faucets
• Heads and holding tank lines
• Washdown pumps, livewells, A/C raw-water loops
• Fuel stabilized, oil changed per manual
• Notes on what was done, for spring

Coolant for closed (freshwater) systems

The closed side of a heat-exchanger-cooled engine uses engine coolant meeting the engine maker's specification, often an automotive-type ethylene glycol coolant. Test its strength with a refractometer before winter and replace it at the interval in the engine manual. Don't let it reach the raw-water side or the water around the boat.

Why non-toxic antifreeze for raw water?

Raw-water antifreeze is discharged overboard when you start the engine in spring. That's why boating guidance specifies non-toxic PG products for the raw-water side, and why ethylene glycol automotive coolant should never be used there. See antifreeze poisoning and disposal.

Common mistakes

• Not running the engine long enough for the thermostat to open. Antifreeze won't reach the whole block.
• Diluting the antifreeze with water left in the system. Drain first or use enough fluid.
• Forgetting the generator.

Recording what you did

Write down which circuits were drained or filled, how much antifreeze was used, and which drain plugs were removed. In spring, that list makes recommissioning safer and prevents a forgotten plug from causing a flood.

Real-world scenarios

Spring recommissioning

1. Reinstall drain plugs and close the petcocks you opened.
2. Check the raw-water impeller and the hoses.
3. Open the seacock.
4. Start the engine and confirm water flow from the exhaust.
5. Check the closed-loop coolant level when the engine is cool.

Short checklist

• Raw side protected
• Closed side tested
• Generator done
• Notes kept

More questions answered

Can I reuse marine antifreeze?

Follow the label and local rules. Most owners use fresh product each season.

Reader questions

Does a jet ski need winterizing?

Yes, per the maker's procedure.

Key point to remember

Follow the engine manual for both cooling circuits.

New to cooling systems? Start with how a car cooling system works. Looking for a product? Search the coolant directory or match your manual's spec with Find my car coolant.

Sources

1. BoatUS: Boater's Guide to Winterizing (PDF)
2. BoatUS: Your boat engine's chillin'